The present invention relates to a phase change memory device and a method for manufacturing the same, and more particularly, to a phase change memory device which can prevent the collapse of the stack patterns of a phase change layer and top electrodes and a method for manufacturing the same.
Memory devices are generally divided into volatile RAMs (random access memory) that lose inputted information when power is interrupted and non-volatile ROMs (read-only memory) that can maintain the stored state of inputted information even when power is interrupted. Some well known volatile RAMs include DRAMs (dynamic RAM) and SRAMs (static RAM). A well known non-volatile ROM includes a flash memory device such as an EEPROM (electrically erasable and programmable ROM).
Although DRAMs are excellent memory devices, DRAMs require a relatively high charge storing capacity. Since the surface area of an electrode of a DRAM must be increased, it is difficult to accomplish a high level of integration for DRAMs. Due to the fact that two gates are stacked on each other for many flash memory devices, high operation voltages are needed as compared to a power supply voltage. According, a separate booster circuit is often needed for flash memory devices in order to generate the necessary voltages required to perform write and delete operations. Further, high level of integration of flash memory devices is also difficult to accomplish.
Much interest has been made in an effort to develop a novel memory device that exhibit simple configurations and that can more easily achieve a high level of integration while retaining the many of the desirable characteristics of non-volatile memory devices. Phase change memory devices promise to realize many of these desirable features.
In the phase change memory device function on the basis of the fact that a phase change can occur in a phase change layer interposed between a bottom electrode and a top electrode. This phase change is associated with a reversible transformation between a crystalline state and an amorphous state brought about by a current flow between the bottom electrode and the top electrode. Accordingly information can be stored in a memory cell of a phase change memory device by measuring the resistances because the specific resistances between the crystalline state and the amorphous state are different.
Phase change memory device often incorporate a chalcogenide layer composed of such material such as germanium (Ge), stibium (Sb), sulfur (S), selenium (Se) and tellurium (Te) is employed as a phase change layer. As a current is applied, the phase change layer undergoes a phase change transistion induced by heat, that is, Joule heat, between the amorphous state and the crystalline state.
Accordingly, in the phase change memory device, the specific resistance of the phase change layer in the amorphous state is often times higher than the specific resistance of the phase change layer in the crystalline state. In a read mode, by sensing the current flowing through the phase change layer, it can be determined whether or not the information stored in a phase change cell has a logic value corresponding to a ‘1’ or a ‘0’.
Conventional phase change memory device are realized by sequentially depositing a phase change material layer and a top electrode material layer on a bottom electrode and then etching the top electrode material layer and the phase change material layer so that top electrodes and a phase change layer are formed. The stack patterns of the phase change layer and the top electrodes have a vertical linear shape.
In the conventional phase change memory devices, the stack patterns of the phase change layer and the top electrodes are formed having a length that extends from a sense amplifier over about one thousand cells. In the convention phase change memory devices, when the stack patterns are patterned to have a vertical line width below 150 nm, a problem can arise that causes the stack patterns to be prone to collapsing.
This collapsing problem can be minimized or avoided by patterning the top electrode and the phase change layer on cells that have the shape of a pillar. Nevertheless, in this case, because four regions corresponding to the peripheral surfaces of the phase change layer are left open, the composition of the phase change layer is also likely to change in the open regions, According the programming current distribution may have to be widened, and as a result the sensing margin decreases.
Therefore, in order to improve and increase the reliability and the manufacturing yield of the phase change memory device, it is necessary to reduce the etch loss of the phase change layer and prevent the collapse of the stack pattern.